This invention relates to torque limited brakes, and more particularly to brakes that are torque limited through mechanical rather than electronic sensing and control means. While the invention will be described with relation to commercial aircraft, it will be understood by those of skill in the art that the principles of the present invention can be applied to brakes on other vehicles besides aircraft.
As a vehicle, such as an airplane, is braked to a stop, such as upon landing or during an aborted takeoff, ground reaction causes the wheels to develop a torque which is passed through the brake rotor and stator to the strut carrying the wheel or wheels. Therefore, it is necessary to design and build the landing gear and the structure attaching it to an aircraft strong enough to react against the greatest amount of torque that can be produced by the brakes without allowing any damage to the landing gear or aircraft structure. At the same time, the brake must be designed so that it will provide enough stopping power to bring the vehicle, in this case an aircraft, to a halt in the prescribed distance. Moreover, the level of retarding force for any braking condition should be limited to a value that is a function of the pilot's brake pedal force. In other words, the pilot should have a feel of what the brake system is doing. At times, balancing these concerns can be quite difficult, particularly in the case of certain brake materials, such as in the case of carbon disk brakes. Carbon has a widely-varying coefficient of friction, depending on the environmental surroundings in which it must work. In some instances, the coefficient of friction of some carbon brake materials has been shown to vary by a factor of four, between wet and dry, new and worn, and hot and cold conditions. If the brake is designed to provide the necessary stopping force, even at the lowest possible coefficient of friction, it can well be that the torque that is produced when those same brakes are operating at their highest coefficient of friction, will exceed the landing gear design strength. At the same time, if the landing gear is designed to withstand the maximum torque possible from the brakes when operating in circumstances under which the coefficient of friction is greatest, it is necessary to overdesign the landing gear to a point which in turn affects the overall weight of the aircraft since additional structural material is added to the landing gear structure.
In order to handle this balancing of concerns, brake control systems have been designed with torque limiters that ensure that the brake will not apply more than a predetermined maximum torque to the strut under any circumstances. In this way, the strut can be designed to the predetermined maximum torque value rather than the possible much greater torque values that would be produced if the brake were not torque limited. Some prior art systems for torque limiting brakes of aircraft have utlized electronic sensors to measure the torque being applied by the brake. The sensors trigger the electronic control of hydraulic valves to release brake pressure at the point of the design maximum allowable torque. The electronic anti-skid and torque limiting systems are costly to design and manufacture and require redundant power sources to avoid a single failure mode that would cause simultaneous loss of torque control of all brakes on the airplane. It is even more complicated a task to design an electronically controlled torque limiter system that limits the torque as a function of the pilot's brake force input for metered brake pressure. Use of a mechanical torque limiting system enables the brakes to be used on any aircraft without concern over the electrical circuitry associated with the other aircraft systems, and provides a limited torque valve that is a function of the pilot's preferred brake pressure. Moreover, a mechanical torque limiting device within each brake provides a single failure mode that can affect only one brake on a landing gear with multiple wheels and brakes.
It is therefore an object of the present invention to provide an apparatus for limiting the torque that can be applied to a vehicle by a wheel brake and more particularly to such a torque limiter that operates by mechanical means directly associated with the braking mechanism itself.